1. Field of the Invention
The invention relates to cellular radio telecommunications systems, and in more particular, to the registration of mobile stations within such systems.
2. History of the Prior Art
In cellular radio telecommunications systems in which mobile stations move from one cell to another and from one system to another, it is critically important to be able to identify the current location of a mobile in order to direct calls to it. This has conventionally been accomplished by the process of mobile registration in which a mobile station registers its presence as it enters the service area of a particular mobile switching center (MSC), and that MSC forwards such registration information to the home location register (HLR) of that mobile station. The HLR maintains in its database a record of the current location of the mobile station at all times so that if a call is received by the mobile station at any system within the network, access of the HLR of that mobile will yield its current location so that the call can then be completed to the mobile at that location.
In a cellular radio system, mobile stations maintain communications with the geographically distributed fixed radio base stations within the system on two different radio links: a voice channel assigned to the mobile by the MSC over which voice communication takes place and a control channel over which data communication between the mobile station and the base station takes place. Such data communication includes registration requests by the system, registration access by the mobile, registration confirmation messages by the system, channel assignment signals, and many other messages. Each base station in a system periodically transmits a plurality of messages over its control channel referred to as the Overhead Message Train (OMT) which includes certain values established for each system. These values include a system identification number (SID), which identifies the particular system, a location area identification number (LOCAID), which identifies a subdivision (i.e., location area) within the system within which the base station is located, a registration ID number (REGID), which is essentially a clock signal, and a registration increment signal (REGINCR), which is a value indicative of the time until the next periodic registration which will be required of the mobile for activity supervision purposes. The sum of the values of the REGID and REGINCR equals a next registration (NXTREG) value which is the time value at which the next registration of the mobile is required by the system.
The data received from the system by a mobile station is very important for the maintenance of communication between the two. Mobile stations may contain within them a first memory area within which each parameter received from a system is immediately stored and a second area within which is semi-permanently stored certain parameter values indicative of the system in which the mobile station is currently registered and with which it is communicating. When a particular parameter value is received on the Overhead Message Train, that value is compared with the corresponding semi-permanently stored value to determine whether the mobile is still operating in the system indicated in its semi-permanent memory.
Another basic concept inherent in the maintenance of communication between a mobile station and a cellular radio telephone system is that of "rescan". Because of the criticality of the control data which the mobile station receives from the system, it is necessary to optimize the quality of the communication link connecting the base station and the mobile station. Thus, a mobile station frequently rescans all of the control channels available to it and selects the channel with the highest quality transmission as the one over which it will communicate with the system. When a mobile station is rescanning and selecting from a plurality of control channels being broadcast by base stations which form a part of the same system or subdivision thereof as indicated by identical values for SID and LOCAID, respectively, i.e. base stations being controlled by the same MSC or subdivision thereof, the MSC and mobile station stay in continuous connection with one another. However, problems begin to arise when a mobile station is operating in a border region between two areas served by different MSCs or two subdivisions of the same MSC area. In such cases, a mobile station may become alternately connected to control channels which are associated with different MSCs or subdivisions of an MSC and thereby create confusion as to the location of the mobile within the system.
For example, a mobile located on the border of two systems may be registered in one system, and its registration information stored in its HLR accordingly, then upon deterioration of the signal in the control channel to which it is connected, rescan and be connected to a control channel associated with a different system. Upon receiving the OMT information from the new system, the mobile would detect new system parameter values and would then attempt to reregister in that system. To do that, the mobile would start a process of rescanning all control channels. It might select a control channel on the old system and send the registration message to that old system. That registration information would then be updated in its home location register, and the registration confirmed by the old system. Thereafter, after receiving a registration confirmation message, this mobile may again rescan and be connected to a control channel on the new system. Upon receipt of the registration confirmation message, the mobile station would have stored in its semi-permanent memory the system parameter values of the new system as detected prior to the attempt to register. This leaves the mobile lost within the system since any new calls received for that mobile would be directed to the old system, with which it is now registered, while it is actually in communication with a base station located within the new system.
The system of the present invention obviates the problems associated with mobile registration in regions near the border of two or between more contiguous systems or contiguous subparts of a system and prevents such confusion.